1. Field of the Invention
The present invention relates to a process for the treatment of heavy oils, including crude oils, vacuum residue, tar sands, bitumen and vacuum gas oils using a catalytic hydrotreating pretreatment process. More specifically, the invention relates to the use of hydrodemetallization (HDM) and hydrodesulfurization (HDS) catalysts in series in order to improve the efficiency of a subsequent coker refinery.
2. Description of the Related Art
Hydrotreating is useful for the purpose of improving heavy oils. The improvement can be evidenced as the reduction of sulfur content of the heavy oil, an increase in the API gravity of the heavy oil, a significant reduction in the metal content of the heavy oil, or a combination of these effects.
The availability of light sweet crudes is expected to diminish in the future as the production of oil becomes increasingly difficult and greater reliance is placed on tertiary and other enhanced recovery techniques. Heavier crudes and sour crudes will take on greater importance in overall hydrocarbon production and the upgrading of such crudes into fuels will become increasingly important. So called heavier crudes contain a larger proportion of heavy and sour material such as high boiling vacuum residue fractions. In addition to the decreasing quality of the crudes and their derived heavy oils, specifications for on-road and off-road fuel will become increasingly more stringent due to environmental legislation around the world. These heavy crudes require deep conversion and refining into lighter and cleaner components through costly techniques, which normally employ high pressures and temperatures.
These conversion techniques compete in terms of their associated capital expenditure and operational expenditure and can range from hydroconversion, such as high pressure ebullating bed conversion to thermal techniques, such as delayed coking. In the case of each, their integration into the refinery can be a costly addition and generally the lower the API of the crude oil, the more constrained the unit will be due to higher associated metals, asphaltene, sulfur and nitrogen. Any technology, method or refinery flow scheme to help increase the profitability of such process units, and therefore, upgrading techniques will prove a significant enabler for the refiner; allowing the processing of heavier and/or sourer crudes, and therefore, positively impacting the upgrading margin.
One of the main limiting factors for hydrotreating units is catalyst deactivation. As the heavy oil feedstock being treated becomes heavier, i.e. has a lower API Gravity, the complexity of the molecules increases. This increase in complexity is both in the molecular weight and also in the degree of unsaturated components. Both of these effects increase the coking tendency of the feedstock, which is one of the main mechanisms of deactivation of the catalyst.
Another aspect of the feedstock leading to deactivation of catalyst is metal content present in the heavy crude. These metals are normally present in the form of porphyrin type structures and they often contain nickel and/or vanadium, which have a significant deactivating effect on the catalyst. Similar to coking tendency, the metal concentration of the heavy oil feedstream increases with decreasing API gravity.
Pre-refining of crude oil would provide a significant advantage for downstream process units. In particular, the removal of metals as well as reduction of aromatics and the removal of sulfur would substantially improve the performance of subsequent coking units.
As the refining industry increasingly processes high sulfur, low API crudes, catalyst deactivation will become a critical path problem, thereby decreasing the on-stream cycle length and therefore increasing the cost of processing, which negatively impacts process profitability. Advances in the treatment of heavy oil with respect to a reduction in catalyst deactivation will therefore be of paramount importance to the refining industry in future years.
In addition to the above challenges, global diesel demand is forecasted to increase in the coming years due to the dieselization trend, equaling global demand for gasoline in the near future and surpassing this demand thereafter. The inherent content of the gas oil in crude oils is limited, and conventional, conversion techniques, such as hydrocracking, that are used to increase the diesel yield by conversion are expensive. There is a need to provide a process for heavy oils that will increase diesel production in a cost-effective manner to meet market demands.